There are a number of geometrical structures made of conducting or dielectric materials that can perform the function of a microwave filter. Such resonators are often used as basic building blocks in bandpass filters for use in the telecommunications field.
A commonly used type of resonator in such situations is the microstrip resonator, which, as its name implies, takes the form of an elongated strip. The strip is supported by a dielectric substrate, which has a ground-plane mounted on its backside.
One difficulty with existing microstrip resonators is that, in use, the current is not evenly distributed within the cross-sectional area of the resonator. In particular, such resonators tend to have peak currents at the edge of cross-sectional boundaries. While careful design and optimization can prevent this from becoming a problem, it also means that the resultant resonator is a relatively large structure.
The problem is exacerbated where superconducting materials are used because the performance of superconducting materials degrades once a critical current is exceeded, which can occur because of the high edge currents. This means that, again, the resonant component must be made relatively large.
Some of these problems can be ameliorated by using a circular resonator. However, altering the diameter of such resonators to change the frequency response causes the frequency of all the resonant modes of the resonator to change proportionally.